For many crops that are planted from seeds, the depth in the ground at which the seeds are deposited can be a critical factor in the economical production of the crop. The economies follow from the accuracy with which the depth of penetration of the seed deposition tools into the soil can be maintained.
The height of the tool-bar can be set, by the farmer, by setting the height of the tool-bar relative to a wheel or wheels. Adjustable travel-limit stops, front and rear, can assist the farmer in getting the operational height of the tool-bar just right, relative to the wheels. The correct depth having been set, it is necessary for the tool bar to heavy enough to maintain that depth of penetration, even though the density of the soil might vary. If the tool-bar were too light, a local hard area of soil might cause the tool-bar to rise, and the depth of penetration to be lost. The supplementation of the weight of the tool bar by the press-down force ensures that the depth of penetration as set by the height of the tool bar relative to the wheels, is necessary to ensure the tools do not ride up over areas of harder soil.
The ground-penetrating tools are mounted in rows, the rows being spaced apart one behind the other. When the implement is being operated over uneven ground, i.e. over ground that can contain hills and valleys, a problem can arise over maintaining the press-down force at a constant magnitude.
The front row of tools are generally maintained accurately enough at the correct depth, because the front row of tools lies close to the rear of the tractor, seeder, or other heavy towing apparatus, and it is easy to transmit a substantial press-down forced to the front row of tools. But it is considerably more difficult to transmit a good push down force to the rear row of tools, and to keep that push-down force on the rear row constant as the implement passes over uneven ground.
Of course, sometimes the ground is level, in which case even the most simple design of press-down force arrangement will suffice. But when the ground is uneven, the task of ensuring a constant press-down force during up/down movements of the tool-bar over the uneven ground requires a more sophisticated design.
Some seeds are susceptible to quite small variations in deposition depth: as little as one centimeter or so in some cases.
The tool-bar is mounted from the tractor or seeder by means of a three-point-hitch. The front of the tool-bar can therefore be held at a fixed height relative to the tractor simply by adjusting the bottom link of the three-point-hitch to a particular height, and then locking the three-point-hitch at that height. The strut that sets the height of the bottom link relative to the tractor then serves to hold down the front of the tool-bar. Even when the ground is uneven, it is not so uneven that the front-row penetration would vary significantly. As mentioned, it is not the front but the back of the tool-bar that is difficult to press down.
As the rear of the tool bar can only be held down when a moment is produced about the ends of the rigid 3-point hitch, the use of a spring has been proposed. The spring being so mounted as to exert a moment or couple on the tool-bar, in the direction to urge the rear of the tool-bar downwards. However, if, during operation, the tool-bar should be inclined downwards relative to the tractor, the spring would have to extend, and the load in the spring, and hence the press-down force, would fall. Similarly, if the tool-bar should be inclined upwards, the spring would be compressed, and the press-down force would rise. Of course, this difficulty could be overcome if the load/length rate of the spring were low, whereby the force of the spring changed only a little with changes in length. However, it has been found that the rates of the push-down springs that can be accommodated on seeders and like implements have been too high and a spring with enough effective motion would be too large. Given the kind of unevenness of terrain in which it is desired to plant seeds, it has been found that the push-down force achievable with a spring varies too much.
It has also been proposed to include a rigid top-link to supplement the three-point-hitch. For operation, the bottom link of the three-point-hitch and the top-link are adjusted in length so that the tool-bar lies parallel to the (level) ground. When the tool-bar passes over uneven ground, the rigid top-link ensures that the angle of the tool-bar remains constant relative to tractor. Clearly, that condition does not favor the maintenance of a constant force on the rear row of tools. When a rigid top-link is used bridging of the implement occurs through valleys and the rear tools rise out of the ground over hills.
The rigid top-link systems that have been proposed have sometimes included a hydraulic ram as, or as a component of, the top-link. The purpose of such a ram was to enable the length of the top link to be adjusted; that is to say, a settable travel-limit-stop was incorporated into the ram or link, and the hydraulic action was used to move the link to that set limit, prior to operation. Once the limit was reached, during operation the hydraulic ram served as a rigid strut.
The invention is aimed at providing a constant press-down force on the rear of the tool-bar, whereby the seed implantation depth can be kept accurate over ground that is more uneven than has been possible with prior implements.